1. Field of the Invention
The present invention relates to an optical pumping cesium resonator and laser diode detection.
At the present time, several thousand atomic clocks are in use. However, modern telecommunication, navigation and pin-pointing systems as well as fundamental and applied research in physics requires the use of clocks which are more and more accurate, and in some cases the present performances of atomic clocks are insufficient for satisfying the expressed needs. Further progress is therefore required.
2. Description of the Prior Art
In the most widely used constructions at the present time, the atoms are deflected in a first two-pole magnet creating an intense and inhomogeneous field. Thus spatial separation of the atoms whose energy is an increasing function of the magnetic field (state F=4,m.sub.F .gtoreq.-3) and atoms whose energy is a decreasing function of the field (states F=4, m.sub.F =-4 and F=3, m.sub.F .gtoreq.-3) are obtained. The most interesting states, F=4, m.sub.F =0 and F=3, m.sub.F =0 between which the clock transition is effected belong to one or other group. At the input to a resonating cavity the beam is enriched in one or other of the states F=4, m.sub.F =0 or F=3, m.sub.F =0 depending on the design. In normal operation, the role of the resonating cavity is to permit the transition F=3, m.sub.F =0.fwdarw.F=4, m.sub.F =0. A second magnet similar to the first deflects towards a hot wire detector the atoms which have undergone the clock transition.
In 1950 M. A. KASTLER proposed replacing the detection and state selection magnets by zones of interaction between the atoms and a light beam. The validity of this proposal was verified by P. CEREZ in 1968 on a rubidium jet. The present interest in such optical methods resides in the development of semiconductor lasers. Some of them generate a 0.85 .mu.m, beam, in satisfactory coincidence (after sorting) with the D2 resonance spectral line of the cesium atom.
If state selection is effected using a single laser, the F=4, m.sub.F =0 level may be peopled to the detriment of the F=3, m.sub.F =0, or conversely.
It is possible to use two lasers LD1, LD2 for optical pumping. It is then hoped to transfer the whole of the atoms peopling the 16 sublevels either to the F=4, m.sub.F =0 level or to the F=3, m.sub.F =0 level. The increase of the atom flow in one of the useful levels is favorable to the performances of the clock.
Detection of the clock transmission is effected by observing the fluorescence light emitted by the atoms interacting in the laser beam coming from a third laser diode LD3 tuned to a closed transition with high photon yield per atom.
This optical detection and pumping cesium resonator of the prior art comprises an oven emitting a cesium jet, a resonating cavity situated in the path of the jet, a first interaction zone in which the beams of the two laser diodes LD1 and LD2 interact with the atom so as to create a reversal of population by optical pumping, a third laser diode LD3 emitting a beam which interacts with the atom flow at the output of the cavity, two fluorescence detectors and means for controlling the frequency of the diodes.
But the most delicate point is the optical detection. Great precautions must be taken for stabilizing the frequency of the laser LD3 for the residual fluctuations of its frequency are transformed into fluctuations of amplitude of the detection light. The result is a reduction of the signal to noise ratio on detection of the clock signal and consequently a degradation of the frequency stability of the clock.
There is no advantage in using the fluorescence signal of the detection zone for stabilizing the frequency of LD3 for this latter is low.
In order to overcome these drawbacks, the invention proposes improving the optical detection of the three laser diode cesium resonator. For this, the third laser diode is brought under control in a third interaction zone situated immediately at the output of the oven. A part only of the beam of this third diode is directed towards the second interaction zone as in the case of the three diode resonator of the prior art.
The advantages of the resonator of the invention are the following:
(1) A better frequency stability of the diode is obtained because of the very good signal to noise ratio with which this fluorescence is detected. The frequency control may be rapid, which results in a considerable reduction of the frequency noise of the diode. That avoids having to use for example an intermediate prestabilization loop using a very stable Fabry Perot resonator;
(2) This solution does not disturb the "zero-zero" reversal of useful populations created downstream, in the first interaction zone for a closed transition does not result in population transfer between the hyperfine levels of the fundamental.